This invention relates to a golf ball which can be readily released from a mold at the end of molding and which exhibits excellent flight performance.
In general, golf balls are provided with a multiplicity of dimples of circular plane shape on their surface for the purpose of improving their aerodynamic properties. It is well known that the dimpled golf balls are far better in flight behavior than smooth golf balls free of dimples.
The flight distance of golf balls depends on the initial velocity, drag and lift acting on the ball during flight, spin rate, and other factors such as weather conditions. It is considered difficult to make a theoretical analysis on golf balls with the aim of increasing their flight distance.
For improving the flight performance of the ball except for the initial velocity which is largely governed by the material of the ball, a number of attempts of tailoring dimples relating to the geometrical factors of the ball have been made. Such attempts include, for example, increasing the diameter of dimples, increasing or decreasing the depth of dimples, changing the shape of dimples from circular one to polygonal and other shapes, and increasing or decreasing the number of dimples. For example, the number of dimples is increased as to maximize the proportion of the sum of dimple areas relative to the entire ball surface area (often referred to as xe2x80x9cdimple surface coveragexe2x80x9d), typically to achieve a dimple surface coverage of 65% or more. More or less fruitful results are obtained from these attempts.
Most often, golf balls are molded using a mold comprising a pair of removably mated mold sections. FIG. 4 shows one typical mold 20 including a pair of upper and lower mold sections 20a and 20b which are removably mated to define a spherical cavity 22. The cavity-defining surface of the mold has a negative dimple pattern, that is, is provided with a multiplicity of projections 21 for indenting dimples in the molding material. The mold sections 20a and 20b are divided along a parting line or plane P lying in register with the equator of the cavity 22. After a molding material is molded in the mold cavity to form a ball, the mold is opened and the ball is taken out from the lower mold section cavity.
In this regard, if dimples with a larger diameter and a greater depth are distributed so as to provide an increased dimple surface coverage, as mentioned above, for the purpose of improving the flight performance of the ball, the molding material is molded in such a mold with the dimple-forming projections being captured in dimples. Then, when the ball is taken out from the lower mold section cavity, some dimples are kept captured with dimple-forming projections disposed near the parting line, disturbing the ball removal from the mold. If the ball is forcibly taken out of the mold cavity, some dimples can be broken at their edge, detracting from the outer appearance and flight performance of the ball.
The golf ball mold mentioned just above is manufactured using a master model. The same problem arises when the mold is separated from the master model.
More particularly, current main techniques advantageously employed for manufacturing the mold are hobbing and precision casting techniques. These mold manufacturing techniques require a reversal step from the master model. When dents for forming the dimple-forming projections for molding large deep dimples are engraved in the master model, the reversal step becomes difficult, that is, the removal of the mold from the master model becomes difficult. If the mold is forcibly separated from the master model, the master model can be damaged.
The attempt to distribute large deep dimples on the ball surface at a high population for the purpose of improving the flight performance of the ball encounters a certain limit, failing to achieve fully satisfactory results.
An object of the present invention is to provide a golf ball having large deep dimples distributed on its surface at a high population, which when molded in a mold, is readily released from the mold and which exhibits excellent flight performance.
The present invention provides a golf ball having on its surface a multiplicity of dimples each delimited by an edge. The dimples include a majority of large dimples with a diameter of 3.0 to 4.2 mm and account for at least 65% of the entire surface area of the ball, the latter factor being referred to as xe2x80x9cdimple surface coverage.xe2x80x9d The large dimples (that is, large diameter dimples) have a depth of 0.1 to 0.35 mm, an edge angle of up to 30xc2x0, and a radius of curvature at the edge of 1 to 50 mm.
In one preferred embodiment, the golf ball has a dimple volume occupation of 0.5 to 1.2%, provided that the dimple volume occupation is defined as the sum of volumes of dimple spaces each defined below a plane circumscribed by the dimple edge divided by the entire volume of an imaginary sphere given on the assumption that no dimples are on the golf ball surface.
Even though large deep dimples are distributed on the ball surface at a high population, the golf ball of the invention is readily released from a mold at the end of molding and exhibits excellent flight performance. More specifically, in a golf ball having on its surface a multiplicity of dimples including a majority of large dimples with a diameter of 3.0 to 4.2 mm and providing a percent dimple surface coverage of at least 65%, several advantages are obtained when the large dimples have a depth of 0.1 to 0.35 mm, an edge angle of up to 30xc2x0, and a radius of curvature at the edge of 1 to 50 mm, and preferably a percent dimple volume occupation of 0.5 to 1.2% is established. While large deep dimples are distributed on the ball surface to provide a high dimple surface coverage, the geometry of the dimple edge can be tailored optimum so as to cause no disturbance against removal from the mold. Even when many large deep dimples are distributed near the parting line of the mold, the ball is still improved in mold release, that is, the ball can be smoothly removed from the mold at the end of molding. Additionally, the large deep dimples distributed on the ball surface at a high population exert their aerodynamic effect to a full extent so that the ball exhibits excellent flight performance.